WASHINGTON - Whenever the world's tropical seas warm several degrees, Earth has experienced mass extinctions over millions of years, according to a first-of-its-kind statistical study of fossil records.

And scientists fear it may be about to happen again — but in a matter of several decades, not tens of millions of years.

Four of the five major extinctions over 520 million years of Earth history have been linked to warmer tropical seas, something that indicates a warmer world overall, according to the new study published Wednesday.

"We found that over the fossil record as a whole, the higher the temperatures have been, the higher the extinctions have been," said University of York ecologist Peter Mayhew, the co-author of the peer-reviewed research published in the Proceedings of the Royal Society B, a British journal.

Earth is on track to hit that same level of extinction-connected warming in about 100 years, unless greenhouse gas emissions are curbed, according to top scientists.

A second study, to be presented at a scientific convention Sunday, links high carbon dioxide levels, the chief man-made gas responsible for global warming, to past extinctions.

In the British study, Mayhew and his colleagues looked at temperatures in 10 million-year chunks because fossil records aren't that precise in time measurements. They then compared those to the number of species, the number of species families, and overall biodiversity. They found more biodiversity with lower temperatures and more species dying with higher temperatures.

The researchers examined tropical sea temperatures — the only ones that can be determined from fossil records and go back hundreds of millions of years. They indicate a natural 60 million-year climate cycle that moves from a warmer "greenhouse" to a cooler "icehouse." The Earth is warming from its current colder period.

Every time the tropical sea temperatures were about 7 degrees warmer than they are now and stayed that way for millions of enough years, there was a die-off. How fast extinctions happen varies in length.

The study linked mass extinctions with higher temperatures, but did not try to establish a cause-and-effect. For example, the most recent mass extinction, the one 65 million years ago that included the die-off of dinosaurs, probably was caused by an asteroid collision as scientists theorize and Mayhew agrees.

But extinctions were likely happening anyway as temperatures were increasing, Mayhew said. Massive volcanic activity, which releases large amounts of carbon dioxide, have also been blamed for the dinosaur extinction.

The author of the second study, which focuses on carbon dioxide, said he does see a cause-and-effect between warmer seas and extinctions.

Peter Ward, a University of Washington biology and paleontology professor, said natural increases in carbon dioxide warmed the air and ocean. The warmer water had less oxygen and spawned more microbes, which in turn spewed toxic hydrogen sulfide into the air and water, killing species.

Ward examined 13 major and minor extinctions in the past and found a common link: rising carbon dioxide levels in the air and falling oxygen levels. Ward's study will be presented Sunday at the Geological Society of America's annual convention in Denver.

Mayhew also found increasing carbon dioxide levels in the air coinciding with die-offs, but concluded that temperatures better predicted biodiversity.

Those higher temperatures that coincided with mass extinctions are about the same level forecast for a century from now if the world continues its growing emissions of greenhouse gases, according to the Nobel Prize-winning Intergovernmental Panel on Climate Change.

In April, the same climate panel of thousands of scientists warned that "20 to 30 percent of animal species assessed so far are likely to be at increased risk of extinction" if temperatures increase by about 3 to 4 degrees Fahrenheit.

"Since we're already seeing threshold changes in ecosystems with the relatively small amount of climate change already taking place, one could expect there's going to be severe transformations," said biologist Thomas Lovejoy, president of the H. John Heinz Center for Science, Economics and the Environment in Washington.

University of Texas biologist Camille Parmesan, who studies how existing species are changing with global warming but wasn't part of either team, said she was "blown away" by the Mayhew study and called it "very convincing."

"This will give scant comfort to anyone who says that the world has often been warmer than recently so we're just going back to a better world," Pennsylvania State University geological sciences professor Richard Alley said.